1. Field of the Invention
The field of the present invention is atmospheric air filtration units, and in particular, an air filtration unit having an air pre-filtration unit and pre-filter attachment device associated therewith for abutting the air pre-filtration unit to the air filtration unit.
2. Background
In heating, ventilating, and air conditioning systems (HVAC systems), and more specifically in variable air volume (VAV) HVAC systems, mini-pleat box filters can be used as medium and high efficiency air filters. Mini-pleat box filters may be configured to filter an air flow in varying filtration efficiencies, such as 65%, 85%, 95%, and 98%. For compatibility with most HVAC systems, mini-pleat box filters are generally square or rectangularly-shaped when viewed from an upstream side.
The fiberglass paper or synthetic non-woven filtration media used in mini-pleat box filters are conventionally rectangularly-shaped mini-pleat media packs which are placed in the mini-pleat box filters in pairs to form a V-shaped filter cells. These media packs are sometimes rigid enough that they may be used without the use of separators. Traditionally, separators held the filtering pleats in position and were often made of corrugated aluminum sheets. The non-woven synthetic fiber media packs can be favored in some applications over the separator-type filtering pleats as they can reduce the decrease in pressure (“the pressure drop”) between an upstream side of the mini-pleat box filter and a downstream side. They also can provide more air flow through the filtration media and can expose the full media surface to the airflow.
In multiple stage filtration systems, a panel or pad filter (pre-filter) can be placed upstream of a mini-pleat box filter (main filter) as part of a pre-filtration stage to remove larger particle sizes from an airflow flowing through the HVAC system. This can be useful to prolong the filtration life of the main filter. The pre-filter stage is generally located upstream from the main filter at a sufficient distance so as to prevent the pre-filter from blinding the main filter. If the pre-filter is arranged so closely to the main filter, that it almost touches the ribs of the main filter, it can disturb the flow of air reaching the pre-filter and the main filter, and thus prevent at least a portion of the airflow from crossing through the main filter. This prevention of airflow by the pre-filter, defined herein as “blinding,” can result in higher energy cost and a higher pressure drop over the total filter assembly and reduce the filtration capacity of the main filter.
Some multiple stage filtration systems provide an entirely separate section in the filtration system to accommodate the pre-filter. As a result, a pre-filter holding frame must be added to these filtration systems, and the filtration systems must be made larger as they require more space in order to accommodate the separate pre-filter section. These filtration systems may also require maintenance personnel to change the main filter and the pre-filter in different locations of the filtration system.
Other multiple stage filtration systems that combine the pre-filter and main-filter stages, do so at the expense of filtration efficiency. For example, in one alternative an upstream edge of the V-shaped filter cells is recessed from an upstream face of the mini-pleat box filter to separate the V-shaped filter cells from the filtration media of a pre-filter that is abutted to the upstream face. A disadvantage of this approach can be in some instances that the recessing is done by reducing the size of the V-shaped media packs, which reduces the filtering potential of the mini-pleat box filter.
In another example, the filtration media of the pre-filter can be recessed in areas located opposite of an upstream edge of the V-shaped media packs. However, doing so can lower the filtering potential of the pre-filter since filtration media must be removed to accommodate the recessed areas. Additionally, in the above examples, conventional metal retainers are generally used to hold the pre-filter in place. Use of these retainers can be disadvantageous in certain instances as the metal retainers can fit loosely and easily corrode, producing corrosion dust particles.